1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst. More specifically, this invention relates to feed and catalyst contacting and to catalyst circulation.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to generally by those skilled in the art as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
Despite the long existence of the FCC process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e. yield and selectivity and regeneration operation. Three facets of the FCC process that have received attention are recovery of adsorbed products from the spent FCC catalyst, stripping of gases from regenerated FCC catalyst and initial contacting of the FCC feed with the regenerated catalyst. Improvement in the recovery of hydrocarbons from spent catalyst directly improves yields. Better initial feed and catalyst contacting tends to benefit yield and selectivity. Removing gases from regenerated catalyst facilitates process operation.
Therefore, improved methods are sought for disbursing feed within the catalyst stream while avoiding localized overheating of the feed and achieving thermal equilibrium between the relatively hotter catalyst and the relatively cooler feed. Such methods would reduce the localized overheating of the feed or the severity of the feed heating caused by the large temperature differentials between the feed and the catalyst which both contribute to feed over cracking.
The processing of increasingly heavier feeds and the tendency of such feeds to elevate coke production makes the control of regenerator temperatures difficult. Optimization of feedstock conversion is ordinarily thought to require essentially complete removal of coke from the catalyst. This essentially-complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 weight percent coke. In order to obtain complete regeneration, oxygen in excess of the stoichiometric amount necessary for the combustion of coke to carbon oxides is charged to the regenerator. Excess oxygen in the regeneration zone will also react with carbon monoxide produced by the combustion of coke, thereby yielding a further evolution of heat. Apart from the objective of minimizing dilute phase CO combustion, the increase in coke on spent catalyst results in a larger amount of coke being burned in the regenerator per pound of catalyst circulated.
Heat is removed from the regenerator in conventional FCC units in the flue gas, and principally in the hot regenerated catalyst stream. An increase in the level of coke on spent catalyst will increase the temperature difference between the reactor and the regenerator, and the regenerated catalyst temperature overall. A reduction in the amount of catalyst circulated is, therefore, necessary in order to maintain the same reactor temperature. However as discussed above, the lower catalyst circulation rate required by the higher temperature difference between the reactor and the regenerator will lower hydrocarbon conversion, making it necessary to operate with a higher reactor temperature in order to maintain conversion at the desired level. This will cause a change in yield structure which may or may not be desirable, depending on what products are required from the process. Also, there are limitations to the temperatures that can be tolerated by FCC catalyst without having a substantial detrimental effect on catalyst activity. Generally, with commonly available modem FCC catalyst, temperatures of regenerated catalyst are usually maintained below 760.degree. C. (1400.degree. F.), since loss of activity would be very severe at about 760.degree.-790.degree. C. (1400.degree.-1450.degree. F.). If a relatively common reduced crude such as that derived from Light Arabian crude oil was charged to a conventional FCC unit, and operated at a temperature required for high conversion to lighter products, i.e., similar to that for a gas oil charge, the regenerator temperature would operate in the range of 870.degree.-980.degree. C. (1600.degree.-1800.degree. F.). Restrictions on catalyst circulation can, therefore, have impact on the effectiveness of feed contacting by restricting the circulation of catalyst.
Restricting the catalyst circulated through the reactor side of the FCC process affects more than yield structure of the products. The circulation rate of catalyst to the reactor influences the catalyst circulation rate through the regenerator. A decrease in the circulation of catalyst to the reactor can also lower the overall catalyst circulation rate through the regenerator. The use of additional conduits such as a recirculation line that transfers catalyst from the outlet of the regeneration zone to the inlet of the regeneration zone can reduce the interdependency of catalyst circulation through the reactor and regeneration zone. However, the use of a recirculation conduit complicates regulation of the catalyst circulation through the process and necessitates the maintenance of additional catalyst inventory on the reactor and regenerator side of the process to provide a buffer for variations in catalyst circulation. Thus, the reactor and regenerator usually operate with two interdependent catalyst circulation loops.
There are a number of patents that de-couple the two interdependent loops by returning catalyst recovered from the reactor back to the reaction zone inlet. U.S. Pat. No. 3,679,576 represents one approach to such recirculation of catalyst where spent and regenerated pass together momentarily through a short section of relatively small diameter conduit before contacting the FCC feed. U.S. Pat. No. 3,888,762 shows a variation on such an arrangement where the feed, catalyst from the reactor and regenerated catalyst all come together simultaneously in a riser conduit. These arrangements offer greater flexibility in the circulation of catalyst through the FCC unit and the catalyst to feed ratio, but they do not address the problem of localized feed over cracking and feed heating severity.
Another group of patents U.S. Pat. No. 5,346,613, U.S. Pat. No. 5,462,652, and U.S. Pat. No. 5,565,177 use a mixture of spent and regenerated catalyst to contact feed in an FCC riser or in an arrangement for ultra short feed contacting. U.S. Pat. No. 5,346,613 that discloses a blending vessel for receiving spent and regenerated catalyst and supplying a mixture of spent and regenerated catalyst to a reaction zone and as recycle to the regeneration zone.
More complete stripping of hydrocarbons from the spent catalyst offers an additional means of recovering more useful products from the FCC unit. More complete stripping removes hydrocarbons from the catalyst that are lost by combustion when the spent catalyst enters the regeneration zone. Common methods to more completely strip catalyst raise the temperature of the spent catalyst in the stripping zone as a means of desorbing hydrocarbons from spent catalyst prior to regeneration. One system for heating spent catalyst in the stripping zone employs indirect heat transfer. A more common method of heating spent catalyst in the stripping zone mixes higher temperature regenerated catalyst with the spent catalyst in the stripping zone. U.S. Pat. Nos. 3,821,103 and 2,451,619 describe systems for direct heating of spent catalyst with hot regenerated catalyst.
In addition to increasing hydrocarbon recovery, reducing the carryover of hydrocarbons into the regeneration zone improves the overall heat balance of the FCC unit Hydrocarbons that enter the regeneration zone release additional high temperature heat as they burn in the oxygen atmosphere. Any additional heat release in the regenerator interferes with the regenerator operation by raising temperatures in the regeneration zone or requiring cooling methods to maintain a suitable temperature.
Further to the stripping of spent catalyst from the reactor, there are also advantageous to stripping the regenerated catalyst before it is sent back to the reactor. While hot catalyst stripping of catalyst entering the regenerator will keep potential products out of the regenerator, stripping of the catalyst leaving the regenerator could displace inert gases from void volume of the catalyst to prevent carryover of inert material from the regenerator to the reactor. Accordingly, it is desirable to perform hot catalyst stripping of regenerated catalyst as well as spent catalyst. However, stripping of regenerated catalyst has not been successfully practiced due to problems of catalyst deactivation. Contact of the high temperature regenerated catalyst with steam will thermally deactivate the catalyst and makes regenerated catalyst stripping impractical.
Therefore in summary, feed contacting and yields have been improved by increasing the catalyst to oil ratio in the initial contact of the hydrocarbons with the hot catalyst. In order to increase the catalyst to oil ratio without increasing the heat supply to the catalyst, systems for blending spent catalyst and regenerated catalyst have been proposed which increase the volume of catalyst while lowering the average catalyst temperature. The spent catalyst that is mixed with the regenerated catalyst has still been found to have sufficient activity to contribute substantially to the catalytic cracking of the hydrocarbons. Again, the heat balance problems associated with the processing of heavier feeds directly interfere with the blending of spent and regenerated catalyst by increasing the coke make and raising regenerator temperatures. Accordingly, more effective stripping of the spent catalyst will permit increased utilization of spent and regenerated catalyst and aid in an additional improvement to feed contacting and yields.